1. Field of the Invention
The present invention relates to a power supply device developing a DC voltage and to a technology effectively applied to the detection of current in a switching regulator, for example, to a power supply driver circuit for driving a power transistor constructing a switching power supply device that performs switching control by a PWM (pulse width modulation) control mode to convert an input voltage and outputs the voltage, and to a driver module incorporating the circuit, and to a technology effectively applied to the a switching power supply using the module.
2. Description of the Related Art
In recent years, microprocessors are mounted as system control devices in many electronic units. Further, there is a tendency for the operating frequency of a microprocessor (hereinafter referred to as “CPU”) to become higher and higher and hence a maximum operating current also increases as the operating frequency increases. In many potable electronic units or the like incorporating a CPU, a system is adopted in which an operating current obtained by reducing a battery voltage by a switching regulator is supplied to a CPU. However, in recent years, a switching regulator capable of outputting a low voltage and a large current has been required as a switching regulator for such a microcomputer system.
Hence, a multi-phase power supply system is proposed in which a plurality of switching power supplies are connected in parallel to increase current supply capacity. In such a power supply system, it is necessary to detect a current passing through a coil and to feed back the driving signal of a power transistor of each power supply (phase) so as to pass an equal current through each phase. A technology for detecting a current so as to prevent an excess current passing through a coil has been known conventionally and does not involve high accuracy for detecting a current to prevent this excess current but involves comparatively high accuracy for detecting a current to perform feedback control.
The following systems are thought as systems for detecting a current passing through a coil: for example, a system for detecting the voltage between the terminals of a current detection resistor connected in series to a coil, as shown in FIG. 1; a system for detecting the voltage between the source and the drain of a power MOS transistor in a system using a MOSFET as a power transistor passing a current through a coil, as shown in FIG. 2; and a system (hereinafter referred to as “sensing MOS transistor system”) in which a current detection transistor connected in parallel to a power transistor is connected in series to a resistor and in which a current proportional to the current of the output power transistor is passed through the current detection transistor to detect the voltage between the terminals of the resistor, as shown in FIG. 3. Here, one of publicly known technologies for detecting a current passing through a power transistor by a sensing MOS transistor system is disclosed in, for example, patent document 1.
[Patent document 1] Japanese Unexamined Patent Publication No. 06-180332
The system using a current detection resistor connected in series to a coil, as shown in FIG. 1, presents a problem that power loss is considerably large because a current passing through the coil passes through the current detection resistor just as it is. Further, the system for detecting the voltage between the source and the drain of a power MOS transistor, as shown in FIG. 2, presents a problem that high detection accuracy can not be obtained because variations in the manufacture of a power MOS transistor and variations in detection value caused by temperature fluctuation are large. Still further, it is also thought to use a transistor having a small on resistance for the power MOS transistor in the regulator shown in FIG. 2. However, in this case, for example, when a rated current of 25 A is passed through a power MOS transistor having an on resistance of 2.5 mΩ, the voltage Vd between its source and drain is only approximately 60 mV, which is hard to detect.
Still further, in the sensing MOS system as shown in FIG. 3, the drain voltage of the current detection MOS transistor is made higher by a voltage drop in the sensing resistor. Hence, if the voltage between the terminals of the sensing resistor is not sufficiently small as compared with the voltage Vd between the drain and the source of the power MOS transistor, the current detection MOS transistor and the power MOS transistor are different from each other in bias condition and hence a current proportional to a current passing through the power MOS transistor does not pass through the sensing MOS transistor, which results in reducing detection accuracy. For this reason, a resistor having a sufficiently small resistance needs to be used for the sensing resistor. Then, the voltage between the terminals of the resistor becomes small, which presents a problem that the detection of the voltage is difficult.
In this respect, the patent document 1 discloses a current detection circuit in which a current detection transistor is provided in parallel to an output transistor for passing a current through a coil and in which the source (emitter) voltage of the current detection transistor is made equal to the source (emitter) voltage of the output transistor by the use of an operational amplifier to pass a current of 1/N to be exact. However, the current detection circuit disclosed in the patent document 1 relates to the so-called series regulator that continuously controls the gate (base) voltage of an output transistor to develop a desired output voltage and is different in a target from the switching device of the present invention. Further, the current detection circuit is different also in a way of detecting voltage from the switching power supply device of the present invention. Still further, in the invention disclosed in the patent document 1, when the developed voltage Vout is low, the voltage Vsns developed between the terminals of the current detection resistor is small and hence is hard to detect. To be specific, when the voltage between the source and the drain of the MOS transistor is assumed to be Vds, Vsns=Vout−Vds. Hence, when the Vout is low, a sufficient detection voltage can not be developed.